1. Field of the Invention
The invention deals with methods for bending glass sheets and more particularly those having glass sheets moving in substantially horizontal planes and which have a pressing shaping phase. The invention more particularly applies to the industrial production of car glazings, such as bent and tempered glazings which are usually mounted to the rear and sides of vehicles, or bent glazings, or annealed, laminated glazings, i.e., those provided on one face with a plastic material sheet or assembled in pairs by means of a plastic material sheet and more particularly intended to serve as windscreens.
2. Description of the Related Art
Most car glazings are bent glazings which have to respect particularly severe optical quality standards and very accurately satisfy a predefined principal curvature, both on the edges, e.g., to permit the encapsulation of the glazing in a frame, and on the remainder of the surface because it may be necessary to pass the glazing into a narrow slot or to pair the glazing in order to form insulating double glazings.
Various apparatuses are known for the bending of a glass sheet in a horizontal position, involving pressing between two complementary bending molds, generally a solid mold and an annular countermold. All these apparatuses can be basically classified into two groups, which differ on the basis of the design of the bending station. In the first group, the station is merely located at the end of the furnace used for heating the glass sheets to beyond their softening point, all the bending tools being at ambient temperature with the exception of the parts in contact with the hot glass. The real advantage of this option is that it significantly simplifies the problem of the design of the tools, their setting and their replacement during production changes. However, this advantage is offset by the cooling of the glass throughout the bending operation due to the low temperature tools. Thus, the temperature of the glass, after bending, must generally be sufficiently high to allow thermal tempering. Moreover, the capacity of a glass to deform at a given speed is greatly dependent on its temperature. In addition, the more the glass cools, the less it is possible to bend it. This is especially prejudicial since this capacity for deforming is also inversely associated with the curvature already acquired, so that all the difficulties are accumulated at the end of the operation. Working outside the furnace therefore makes it necessary to overheat the glass sheets in the furnace, which increases optical deformation risks and combines the tooling problems with those associated with the glass and the respecting of its optical quality.
The second group of flat bending apparatuses consists of isothermal apparatuses, where the bending station is placed in a hot, sealed enclosure, where ambient temperature is close to the shaping temperature of the glass, i.e., between 600.degree. and 750.degree. C. Therefore, these apparatuses are also referred to by the term "bending apparatuses in the furnace", even if in reality bending takes place in a heated enclosure and not in the furnace. This option is advantageous with respect to the end product but not the tools used in its production. Thus, it is possible to bend glass sheets with much lower furnace exit temperatures than in the preceding case. It is no longer necessary to overheat the glass in order to compensate for heat losses during bending, which is favorable from an optical quality standpoint. Moreover, the thinness of the glass sheets or films, which is a limiting factor in the case of possible cooling during bending, is not as critical as previously.
However, this advantage is offset by a much more complicated design of the complete shaping apparatus, and in particular the tools. The first problem is that of the transfer of the glass sheet from the furnace to the bending press or, and this amounts to the same thing, that of its discharge, bearing in mind that the choice of a horizontal line is mainly dictated by the wish to avoid all grippers and other instruments acting in a localized manner and which consequently leave behind unremovable marks on the glazing.
A first solution is to supply the press with a suction plate performing a reciprocating movement between the end of the furnace and the press, the glass sheet being deposited on the lower mold, which is translated to a position itself facing the upper bending countermold (EP-A-183,418). However, the accuracy of bending requires a very high degree of control of these different lateral displacements, which at high temperature is difficult to obtain with a high regularity, particularly if an accuracy of approximately 1/10 mm is required, which is the case with the most complex glazing shapes.
Another solution is to extend the conveyor carrying the glass sheets up to the place where pressing takes place. In this case, it has been proposed to raise the glass sheet, e.g. by suction and upward hot gas flows, in order to bring it into contact with the upper mold, which is then raised, which leaves between the glass sheet supply conveyor and the upper mold a space which is sufficient to allow the introduction of a lower frame. After pressing by means of said frame, the glass sheet is again taken up by the upper mold until it is recovered by a tempering frame (EP-A-237,231). In the cold state, another possibility is to pass the lower pressing frame through the conveyor, either by the frame being formed by a series of segments between which can be located the conveyor rollers, or by the rollers having breaks providing a recess for the rest position for the pressing frame, or by the actual rollers being segmented. However, it should be noted that a segmented frame tends to promote the marking of the glass, the sheet not being uniformly supported over its periphery, while the two other variants, i.e., rollers with breaks or segmented rollers, cause enormous construction and design problems with respect to the roller drives, and these problems have not hitherto been solved for operations at high temperatures.
Finally, it is proposed in U.S. Pat. No. 3,869,271 that a shaping process based on vacuum be used, in which the glass is moved through the furnace by an air cushion and then penetrates the shaping zone outside the hot enclosure, where it is supported by a second air cushion surrounded by a pressing frame mounted in an overhanging manner and located just below an upper concave mold. The glass sheet is immobilized above the pressing frame lowered beneath the plane of the air cushion. The pressing frame is raised again and engages the glass sheet against the upper bending mold. To ensure that a large portion of the glass sheet is not without support during the transfer from the first air cushion to the air cushion of the shaping zone, in the interposed zone are provided roller sections for preventing any sag by gravity.
Here again, it must be stressed that the pressing device is in the cold zone. Moreover, in the aforementioned case, the glass sheet is supported in the pressing zone by an air cushion located within the pressing frame. In practice, such an arrangement is only possible if the dimensions of the planar glass sheet do not differ excessively from those of the frame, and therefore from the bent glass sheet, i.e. when the curvature imparted remains limited. In the opposite case, it is necessary to provide another air cushion surrounding the frame, which further complicates the problems of regulating or adjusting the tools, the balancing of the segmented air cushion being particularly difficult. Moreover, the checking of the trajectory of a moving glass sheet on a conveyor is difficult, because two glass sheets following one another can have trajectories which are several centimeters apart. These trajectory variations are difficult to reconcile with the combined requirements of high optical quality and great accuracy of the principal curvature. In order to compensate for these variations, it is admittedly possible to use a recentering member, such as is e.g. described in U.S. Pat. No. 4,233,049, but this further complicates the apparatus.
Moreover, this centering of the glass sheet is only advantageous if the two pressing molds coincide. However, in the aforementioned document, the annular countermold overhangs, whereas the upper mold is suspended. This leads to positioning variations which are difficult to compensate and which would be even more prejudicial if the tools were placed in the hot zone, even without taking account of the trivial, but real, difficulty that accessibility and visual checking are less difficult when the bonding zone is closed by side walls.